The present invention relates to the gasification of organic matter, and more particularly to the gasification of organic solids recovered from wastewater sludge.
Industrial and agricultural processes and municipalities produce large quantities of organic wastes, which may be either relatively dry solids or may be contained in wastewater streams. Historical waste management techniques have included landfill storage of solid wastes and marine disposal of wastewater streams, resulting in severe environmental impact. Additionally, the energy that is stored within such organic materials is not utilized.
There has thus arisen a desire to develop techniques for reducing waste volume while recovering energy from organic waste matter. Processes have been developed to incinerate solid wastes and dewatered wastewater streams. Incineration of municipal sludge has been fairly widespread, but more recently has fallen into disfavor. There is little or no beneficial reuse of the energy in wastes with incineration, and in many cases, a great deal of auxiliary fuel is consumed. In addition, hazardous emissions, such as dioxins and heavy metal compounds, may be produced by incineration
Some conventional gasification processes have addressed difficulties in gasifying dewatered waste solids from wastewater streams by admixing easily combusted, concentrated-energy fuels, such as coal fines, to the waste material. While enhancing the ability to combust the wastes, the requirement of adding coal further reduces overall process efficiency.
As an alternative to conventional gasification techniques, many municipalities treat wastewater and sewage streams to produce a sludge that is suitable for use as an agricultural soil amendment. However, the costs of transporting the sludge from the municipal generation site to agricultural sites, and of spreading the sludge on fields, is high.
The process of the present invention provides a method for gasifying organic waste materials, such as aqueous sludge containing organic solids. The sludge is first dewatered to a solids content of at least 35% to 65%, and preferably 50% by weight. In a preferred embodiment of the invention, dewatering is carried out by centrifugation, followed by heating in a microwave heat exchanger, and then further dewatering by passing the sludge through a screw press. The dewatered solids are then at least partially pyrolyzed, producing gases, tars, char and unpyrolyzed solids. These pyrolysis products are then further gasified by anoxically exposing the products to high intensity microwave radiation in a microwave gasification reactor, producing carbon containing fuel gases and char. The fuel gases are then separated from the char. The fuel gases are cleaned, compressed and combusted to generate electricity and process heat by a cogeneration process. When the char is burned, the inorganic portion is liquified and quenched to produce vitrified ash. In a preferred embodiment of the invention, char combustion and ash vitrification is carried out in a cyclonic furnace. The vitrified ash may be used as an inert additive for manufacture of products such as roofing tiles, paving asphalt, cement bricks or road concrete.
The present invention thus provides a highly efficient process and system that concentrates solids in wastewater sludges, greatly reduces the volume of those solids, and recovers a significant portion of the energy stored in the waste. The use of microwave gasification is, found to be a highly efficient method of gasification.